Anaerobic bacteria are normal flora of the oral cavity and the gastrointestinal tract.

While oral anaerobic flora are mostly Gram-positive organisms such as anaerobic cocci and Peptostreptococcus spp., the principal anaerobic intestinal flora are Gram-negative bacilli such as Bacteroides fragilis and Fusobacterium spp.

Gram-positive oral anaerobes are widely covered by most of the orally-available agents, including penicillin. However, antibiotic activity against the most common intestinal anaerobic bacteria, Bacteroides fragilis group, is variable.

Anaerobic coverage is indicated in a variety of infectious processes, including but not limited to aspiration pneumonia, intra-abdominal infection, gynecologic infection, and diabetic foot infection.

Double anaerobic coverage is not necessary and puts the patient at risk for additional drug toxicities.